Broad band electroacoustic transducer



July 14, 1959 F. R. ABBOTT BROAD BAND ELECTROACOUSTIC TRANSDUCER Filed Dec. 22, 1955 2 Sheets-Sheet l INVENTQR. FRANK n. ABBOTT BY 4/ 6 I A7 ORNEYS Fig. 3

July 14, 1959 F. R. ABBOTT 2,895,062

BROAD BAND ELECTROACOUSTIC TRANSDUCER Filed Dec. 22, 1955 I 2 Sheets-Sheet 2 Fig; 4

INVENTOR.

FMNK R ABBOTT W A romvsrg Patented July 14, 1959 ice BROAD BAND ELECTROACOUSTIC TRANSDUCER Frank R. Abbott, San Diego, Calif.

Application December 22, 1955, Serial No. 554,901

4 Claims. (Cl. 310--9.6)

(Granted under Title 35, U.S. 'Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to broadband electroacoustic transducers and more particularly to an electrostrictive ring for driving a concave or convex membrane to improve the acoustic lcoupling from the ring to the surrounding media over a broad frequency range and particularly at low frequencies.

Heretofore, transducers have been made of piezoelectric, magneto-strictive or electrostrict-ive materials without satisfactory coupling means to the surrounding media at low frequencies. These transducers are designed to operate at sonic or supersonic frequencies and depend upon the resonant frequency of the ceramic itself. These frequencies are determined by the geometry and polarization of the materials used and range from 500 cycles to 10,000 cycles. In some instances ceramic elements have been used at the higher frequencies and electromagnetic elements, such as loudspeakers, have been used for the low frequencies.

The transducer comprising the present invention utilizes electroor magncto-strictive rings to deform thin concave or convex membranes in such a manner that the amplitude of mot-ion of the rings is greatly increased over a larger area. Thus, small stiff elements such as barium titanate rings which normally achieve ineffective coupling except at ultra sonic frequencies can be made to serve as powerful acoustic radiators for low frequencies. By employing mass loading of the surrounding medium to obtain resonance at a small fraction of the frequency characteristic of the ceramic itself, the transducer comprising the present invention can be utilized best at 100 cycles and has an effective range of from 40 cycles to 40,000 cycles.

It is therefore an object of the present invention to provide for a transducer having an improved coupling with its surrounding media over a broad frequency range.

Another object is the provision of a membrane coupling between a piezoelectric, magneto-strictive or electrostrictive material and its surrounding media.

Another object is the provision of a ceramic transducer for operation at sub-sonic frequencies.

A further object is the utilization of radial electrodeformation of a stiff ring to produce normal vibration of a concave membrane to improve the acoustic coupling from the ring to its surrounding media over a broad frequency range.

Still another object is the provision of a transducer which increases the effective source strength at low frequencies.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 shows a plan view of the transducer;

2 Fig. 2 is a cross-sectional view taken along the line 2-2 in Fig. 1

Fig. 3 is an enlarged view of a portion of Fig. 2;

Fig. 4 shows radiation patterns of the transducer at different frequencies; and

Fig. .5 shows a modified form of the invention.

Referring now to the drawings wherein like numerals designate like parts throughout the several views, a ring 11 of electro strictive material such as barium titanate has secured to it metallic membranes 12 and 13. These are bonded to the ring with an insulating cement 15 and have lips 14, 16 extending around the periphery thereof. To lip 16 is attached an outer electrode 17 of the ceramic ring. An inner electrode 18 provides electrical continuity between the inner surface of ring 11 and membrane 12. Electrical leads 19 and 21 transmit energy through the metallic membranes 12, 13 to electrodes 17, '18. The membranes 12, 13 are deformed to either concave or convex form, the initial deformation largely determining the acoustic impedance conversion factor for the transducer.

In operation a variable radial electrical field is applied by the leads to the radially polarized ring of electrostrictive ceramic material. The electrical stress applied to the ring causes a radial expansion which in turn stretches the membrane to remove the preformed concavity. A small amount of radial expansion greatly reduces the depth of the deformity. To be explicit, if a metal membrane has a depth of concavity h when cemented to the edge of a ring of diameter D and the diameter is increased to D plus AD, then the concavity is reduced to Thus, a metal membrane with inch static depression h afiixed to a 10 inch diameter D ceramic ring will rise at the center an amount of 10 AD when the diameter increases by AD. Thus, the normal velocity of the membrane becomes ten times the radial velocity of the ring. The effective strength of any small energizing source is the integral of the normal velocity (of radial expansion of the ring) times the area of the membrane covering the ring. Thus, the membranes greatly expand the source strength at low frequencies. While at high frequencies, tests show that the membranes have a decreasing effect, they still contribute to the effectiveness of the device as an acoustic radiator due to more effective acoustic coupling.

In cases where convex rather than concave membranes are preferred, it is advantageous to have a positive static pressure within the transducer. Fig. 5 shows such a device with valve 20 for regulating the amount of gas pressure.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In a transducer, a piezoelectric ring having an inner surface, an outer surface and two sides, a first thin metallic membrane secured to one of said sides and having a lip extending over a portion of said outer surface, a second thin metallic membrane secured to the other of said sides and having a lip extending over a portion of said outer surface, an inner electrode providing electrical contact between said first membrane and said inner surface of said ring, an outer electrode providing electrical contact bet-ween said outer surface of said ring and said lip on said second membrane, electrical leads connected to both of said membranes, said ring adapted to expand radii w W 2. In a transducer, a piezoelectric ring adapted to ex- 7 pand radially upon electrical excitation, a thin membrane initially deformed for sub-sonic frequencies attached to said ring, said membrane having a diameter greater than the diameter of said ring defined by points of attachment thereto, and electrical means including said membrane for radially expanding said ring whereby said ring diameter increases and approaches the length of said'membrane diameter.

3. In a transducer, a piezoelectric ring having a uniform thickness spacing parallel side surfaces, a first membrane attached with insulating cement to one of said side surfaces, a second membrane attached with insulating cementto the other of said, side surfaces, said membranes being concave shaped so that their centers are normally spaced apart a distance less than said ring thickness, an electrode connecting the first membrane and the internal bore of said ring, an electrode connecting the second membrane to the peripheral surface of said ring, means for radially expanding said ring to increase the distance between the centers of said membranes.

4. In a transducer, a piezoelectric ring having a uniform thickness spacing parallel side surfaces, a first membrane attached to one ofisaid side surfaces, a second membrane attached to the other of said side surfaces, said membranes and said ring forming a gas tight compartment, a gas under pressure. in said compartment, said membranes being convex shaped and having centers spaced apart under gas pressure a distance greater than said ring thickness, means for radially expanding said ring to vary the distance between said centers of said membranes.

References Cited in the file of this patent UNITED STATES PATENTS 2,386,279 Tibbetts Oct. 9, 1945 2,403,692 Tibbetts July 9, 1946 2,477,596 Gravley Aug. 2, 1949 2,487,962 Arndt Nov. 15, 1949 2,607,858 Mason Aug. 19, 1952 

